Method to increase the oil production from an oil reservoir

ABSTRACT

A method to increase the production of oil from an oil reservoir is described. The method includes injecting a magnetic or magnetostrictive material through an oil well into the oil reservoir, vibrating the material with the aid of an alternating electric field and removing oil from the oil well.

BACKGROUND OF THE INVENTION

The present invention is related to a method to increase the oilproduction from an oil reservoir.

Recovery of oil from oil reservoirs under electrical stimulation hasbeen described for instance in NO 161.697 and U.S. Pat. No. 4,884,594 aswell as U.S. Pat. No. 5,282,508 corresponding to NO. appl. 922581.

The above patents is related to an enhanced oil recovery methodcurrently known as the Eureka Enhanced Oil Recovery (EEOR) principlewhich is an enhanced oil recovery method specially designed forland-based oil fields. The principle is based on electrical and sonicstimulation of the oil-bearing strata in such a manner that the oil flowis increased.

This is done by introducing special vibrations into the strata. Thesevibrations will be as identical to the natural frequency of the rockmatrix and/or the fluids as possible.

The vibrations give rise to several effects in the fluids and remaininggases in the strata. They decrease the cohesive and adhesive bonding, aswell as a substantial part of the capillary forces, thereby allowing thehydrocarbons to flow more easily in the formation.

The vibrations that propagate into the reservoir as elastic waves willchange the contact angle between the rock formation and the fluids,thereby reducing the hydraulic coefficient of friction. This allows afreer flow towards the wells where the velocity increases and creates agreater pressure drop around the well. The elastic waves give rise to anoscillating force in the strata, which results in differentaccelerations because of the different densities in the fluids. Thefluids will “rub” against each other because of the differentaccelerations to create frictional heat, which in turn reduces thesurface tension on the fluids.

The vibrations also release trapped gas that contributes to asubstantial gas lift of the oil. Furthermore, the oscillating forcecreates an oscillating sound pressure that contributes to the oil flow.

Heat is supplied to the reservoir to maintain and, at the same time,increase the pressure in the oil field when its natural pressure hasbeen reduced. The heat is supplied both as frictional heat, from thevibrations, and also as alternating current into the wells. Theelectrical transmission capabilities always present in an oil fieldallow the alternating current to flow between wells to make thereservoir function in a manner similar to an electrode furnace becauseof resistance heating.

The heating causes a partial evaporation of the water and the lightestfractions of the hydrocarbons and remaining gases in the oil.Furthermore, the alternating current causes the ions in the fluids tooscillate and thereby creates capillary waves on the fluid interfacesand thus reduces the surface tensions, a phenomenon we have named “Thein situ Electrified Surfactant Effect (IESE).

The heat created from the electrical stimulation and from the vibrationsreduces the viscosity of the fluids.

The oil flow acts as a cooling medium that allows a greater energydensity from the vibrator and the electricity supplied to theoil-producing wells.

A number of possibilities exist for the use of electricity to heatoil-bearing formations. These methods can be classified according to thedominant mechanism of thermal dissipation in the process. The linefrequency plays a decisive role in how the electrical (andelectromagnetic) energy is converted to heat. Dielectric heatingprevails in the high-frequency range from radio frequencies to microwavefrequencies. The dipoles formed by the molecules tend to alignthemselves with the electrical field. The alternation of this fieldinduces a rotation movement of the dipoles with a velocity proportionalto the alternation frequency. The molecular movement can be intenseenough to produce considerable heat. A popular application of thisprocess is in microwave ovens. Another possibility is inducing heatingwhere the alternating electric current flows through a set ofconductors, inducing a magnetic field in the medium. The variations ofthe magnetic fields, in turn, induce a secondary current whosecirculation in the medium creates heat. This work is confined to theresistive heating process, which is the major mechanism when DC orlow-frequency (up to 300 Hz) alternating current is used.

The electrical heating of a reservoir formation was used to enhance oilproduction as early as 1969, when an experiment in Little Tom, Tex., wassuccessful. The production of four wells had increased from 1 bbl/d(0.16 m³/d) to an impressive average of 20 bbl/d (3.18 m³/d) for theexperiment, which included wellbore fracturing. The method subsequentlyattracted the attention of an increasing number of investigators andengineers, and their field tests were reported within a few years. Thefirst academic work on resistive heating was by El-Feky in 1977. Hereported on the development and testing of a numerical model that wasbased on implicit-pressure, explicit-saturation formulation over atwo-dimensional rectangular grid. Experimental data came from alaboratory model consisting of a five-spot water flood. The electricalconcept was later coupled to water-injection processes to derive theso-called reservoir-selective-heating method.

Until 1986, the few existing reservoir simulators for the electricalenhanced process relied on explicit treatments to determine saturation,voltage, temperature, and pressure. Killough and Gonzales presented afully explicit three-dimensional multicomponent model in 1986 that wascapable of handling water vaporization. The authors focused on the ideaof flood patterns for the heating water. In 1988, Watttenbarger andMcDougal used a two-dimensional simulator to investigate the majorparameters affecting the production response to electrical heating. Theyconsidered the steady-state regime to obtain a simple method forestimating the production rate.

Thomas Gordon Bell describes electroosmosis by electrolinking two ormore oil wells In U.S. Pat. No. 2,799,641. William C. Pritchet describesa method and the apparatus for heating a subterranean formation byelectrical conduction in U.S. Pat. No. 3,948,319. The method describesthe use of alternating or direct current to preheat the formation.

Lloyd R. Kern describes the use of electricity to “melt” hydrates (atypical methane hydrate have the chemical formulation CH₄H₂O) formed intypical arctic shallow formations.

E. R. Abernathy discusses the use of electromagnetic heating of the areanear an oil well. [REF Journal of Canadian Petroleum Technology,July-September 1976, Montreal]

A. Herbert Harvey, M.D. Arnold and Samy A. El-Feky report a study of theusability of an electric current in the selective heating of a portionof an oil reservoir that is normally bypassed by injected fluid. [REFJournal of Canadian Petroleum Technology, July-September 1979, Montreal]

A. Herbert Harvey and M.D. Arnold describe a radial model for estimatingheat distribution in selective electric reservoir heating. [REF Journalof Canadian Petroleum Technology, October-December, 1980, Montreal]

Erich Sarapuu describes a method in underground electrolinking by animpulse voltage to make cracks in the formation in U.S. Pat. No.3,169,577.

The contribution of the different liquids to the pressure buildupdepends on the original pressure, temperature and liquid/gasrelationship in the reservoir. In a reservoir with low gas content,pressure and temperature, the main contribution to the increasedpressure comes from evaporation of water and lighter crude fractions,and from thermal expansion of the gas.

The temperature and pressure increase occur not only in the vicinity ofthe well, but also between the wells, depending on the paths of theelectrical potential between the well.

The energy input for each well depends on the oil flow and the settemperature in the bottom zone. This means that for a particularelectrode (casing) temperature, which depends on the equipment, thepower input depends on the cooling effect of the oil produced. Thegreater the oil production, the greater the energy input possiblebecause the increased heat at the well area is drained away by the oilproduced. If no oil is produced, the heat flow into the formation fromthe well would take place by heat conduction only.

SUMMARY OF THE INVENTION

The invention is drawn to a method for increasing the oil productionfrom an oil reservoir. A magnetic or magnetostrictive material isinjected into the end oil reservoir and then the material is vibratedwith the aid of an alternative electric field. Oil is then drawn fromthe same oil reservoir from the same well in which the magnetic ormagnetostrictive material was injected. The vibrations created in theinjected material can be changed by changing the frequency of theapplied electric current passed into the reservoir.

BRIEF DESCRIPTION OF THE FIGURES

The invention may be more completely understood by considering thedetailed description of various embodiments of the invention whichfollows with the accompanying drawings.

FIG. 1 is a schematic drawing of a flow chart reflecting the basiccomponents of an in-situ electrical surfactant effect that is known inthe art.

FIG. 2 is a schematic drawing representing physical components of theEureka oil recovery principle as known in the art.

FIG. 3 is a flow chart diagram of the Eureka oil recovery principle asknown in the art.

FIG. 4 is a schematic illustration reflecting mass acceleration due tooscillating elastic sound waves according to the Eureka oil recoveryprinciple, as known in the art.

FIG. 5 is a schematic illustration of continuous streams of oil capableof flow that are formed out of oil droplets when the droplets areexposed to vibrations.

FIG. 6 shows the results in graph format for both pulse and continuouswave mode excitation as a function of the sound intensity.

FIG. 7 is a graphical representation of hydrodynamic pressure and energylosses as a function of distance and viscosity of a fluid.

FIG. 8 is a graphical comparison of secondary water depletion versus theEureka stimulation of a three-dimensional artificial reservoir as afunction of pore volume injection.

DETAILED DESCRIPTION OF THE FIGURES

In the EEOR process, we apply a low-frequency alternating current of 100to 500 V, depending on the resistance in the reservoir. This electricalenergy is delivered as a resistive heating process. The energy from thewellbore will be delivered logarithmically according to the formulaE=U1*1nr/1nR.

During the development of the Eureka-process, we observed an immediateincreased oil recovery using electricity and vibrations before one couldexpect a thermal effect. We concluded that this increase may be causedby the ions in the fluids oscillating in response to the electricalmodulation.

Ions at the fluid boundaries can be polymerized to a thickness ofseveral molecules. This means that the ions are more or less linked orlined up in with the electrical charge in one direction, and this is oneof the effects that creates the surface tension in a fluid.

When applying an electrical field, E, to a charged particle, thisparticle will experience a force F given by

F=qE  (1)

If the particle has mass m, it will experience an acceleration, which,according to Newton's second law is

a=F/m=qE/m  (2)

Let us look at a charged particle in a region of uniform electric fieldwith the magnitude and direction of E the same everywhere. This regionof electric field may be approximated in practice by maintaining equalbut opposite charges on two conducting plates.

Equation (1) asserts that a charged particle in a uniform fieldexperiences a constant acceleration. Then all the kinetics, dynamics andenergy relationships associated with particles undergoing constantacceleration apply to a charged particle in a uniform field. Forexample, if we assume a constant electric field of magnitude E in the ydirection, a particle of mass m bearing a charge q in that field has aconstant acceleration a_(y)=qE/m. The kinetics equations for constantacceleration apply:

y=y _(o) +y _(oy) t+qEt ²/2m  (3)

v _(y) =v _(oy) +qEt/m  (4)

v _(o) ² =v _(oy) ²+2qE/[m(y−y ^(o))]  (5)

Let us choose the x direction as horizontal and the y direction asvertical, and let the initial position of the charged particle be at thecoordinate origin.

The initial velocity v_(o) of the charged particles has components v^(o)_(x)=v^(oy)=v^(o)/2^(0.5). Because E is in the positive y direction, theconstant acceleration of a negative charged particle is in the negativey direction. Because the charged particles experiences no accelerationin the x direction, we may adapt Equation (3) for both the x and the ydirection of the charged particle at any time t::

x=v _(ox) t  (6)

y=v _(oy) t−qEt ²/2m  (7)

When the charged particle has returned to its original height, y=0,Equations (6) and (7) may be written:

d=v _(ox) t  (8)

v _(oy) =qEt/2m  (9)

Eliminating t from Equation (8) and using v_(x)=v_(oy)=v_(o)/2^(0.5)gives

E=2mv _(ox) v _(oy) /qd=mv ² /qd  (10)

The kinetic energy of a charged particle after it has been released fromrest in a uniform field that is in the positive y direction is E=Ej.Suppose the particle has a mass m and charge q. When the particle hasmoved from the origin to a position y, the particle will have acquiredkinetic energy K=mv²/2. Equation (5) provides that vy²=2qEy/m, so thatthe particle has a kinetic energy of

K=m/[2(2qEy/m)]=qEy  (11)

at position y.

The kinetic energy of the particle may also be calculated using thework-energy principle, and will be the same. The work done by theresultant force on a particle is equal to the change in the kineticenergy of particle. When a particle with charge q moves from the originto a position y, the work on that particle by a constant force qEj isqEy. Thus the change in the kinetic energy of the particle, andtherefore its kinetic energy at the position y, is K=qEy, a valueidentical to that of Equation (11).

Now this is a charged particle in a uniform field. Using the EEORprinciple, we have an oscillating electrical field and thereby we willachieve an oscillating motion of the particles (ions) in accordance toEquation (2). If we look at a round drop with ions (FIG. 4-3), the ionsat the surface will encounter the electric field at a different time andbe accelerated in a different direction because of the curvature on thedrop. Because of the opposite charges of the anions and the cations, theions will also be accelerated in opposite directions. These oppositeaccelerations of the particles are probably what gives rise to thecapillary waves on the interfaces mentioned earlier.

The total energy delivered to the ion concentration at the surfacecreating the capillary waves has to be able to actually break the freesurface energy of the liquid, i.e., it has to exceed the free surfaceenergy.

We believe that breaking the surface tension creates an effect similarto that of a chemical surfactant reducing the same tensions and reducingthe “clogging effect” of water droplets in the pore necks.

The electrical stimulation of the well can be arranged in different waysdepending on the actual well configuration. The energy is delivered froma step-wise regulated transformer with a complete set of instrumentationto monitor the current, voltage and energy delivered over each phase.The power to the wellheads is delivered by cables normally buried 30 cmunder the ground. The cables at the wellhead are connected to thepower-carrying cable down the well, which can be:

1. By insulated casings stripped at the “pay” zone—the cables aredirectly connected to the casing at the wellhead.

2. By under-reaming of the existing casing above the “pay” zone—thecurrent is delivered either by a downhole cable to the casing at the payzone or via the tubing when using insulated centralisers.

3. By “antenna wells” directly on the casing at the wellhead.

In any of these arrangements, electrical safety is maintained by normalprotection of any current-carrying parts. The wellhead itself isprotected by a fence.

Each site is designed individually and an installation can consist ofnew drilled wells, under-reamed wells and existing wells used as“antennas.”

A typical arrangement is shown in FIG. 2. The total effect of theelectrical stimulation is illustrated in FIG. 3.

The challenges or possibilities related to relatively weak elastic wavestimulations of a reservoir were first addressed by researchers in theUnited States and the Soviet Union in the late 1950s. The activitypeaked in the early 1970s in the United States and continued in the1970s and 1980s in the Soviet Union. Most of the work in this area hasbeen conducted by Soviet research and industrial institutions, primarilythe Institute of Physics of the Earth of the U.S.S.R. Academy ofSciences, the Krylov Institute of Oil and Gas (VNNII), and the Instituteof Nuclear Geophysics and Geochemistry (VNNIIYaGG) (currentlyVNNIIGeosystem), all in Moscow, as well as the Special Design Bureau ofApplied Geophysics of the Siberian Branch of the U.S.S.R. Academy ofSciences in Novosibirsk. In addition, this review includes an outline ofthe results published in the Russian literature not readily accessibleto western researches.

Interest in the effect of elastic waves on oil and water, oil, and gasproduction dates back to observations made to find the correlationbetween water-well levels and seismic excitation from cultural noise andearthquakes. A sharp change in water level in a 52-m-deep well inFlorida caused by nearby passing trains and a remote earthquake (Parkerand Stringberg, 1950) was observed. The fluctuations caused by trainswere approximately 1-2 cm and were comparable with the fluctuationcaused by the earthquake. Unfortunately, the distance from the source isnot reported in this paper. The low-frequency fluctuations were causedby changes in atmospheric pressure and earth tides. The same workreported a 1.4-m fluctuation in the water level at a different well inFlorida attributed to an earthquake originating 1200 km away.

Barbarov et al. (1987) studied the influence of seismic waves producedby a vibroseis-type source at excitation frequencies of 18-35 Hz onwater levels in wells 100-300 m deep. Kissim (1991) summarised theresults of these experiments. The seismic waves produced water-levelfluctuations of 1-20 cm. In addition to these short-term fluctuations,longer term changes in water level induced by a seismic source wereobserved for periods up to five days. The presence of resonancefrequencies to which aquifer responded sharply is noted. Barbanov et al.(1987) observed that the effects of vibroseis-type sources of aquiferswere comparable to those of teleseismic earthquakes. A sharp fluidpressure response in California aquifers associated with the Landersearthquake was reported recently (Galloway, 1993). Observations fromthis earthquake show a 4.3-fold increase in the fluid pressure thatdecayed exponentially for several days to weeks. It is worth noting thatthe decay rate is consistent with the one observed by Barbanov et al(1987) after vibratory action.

The extensive study of hydrogeological effects produced throughout theworld by the Alaska earthquake of 1994 revealed a significant influenceon fluid level in wells (Vohris, 1968). The earthquake was purported tohave produced observed changes in well levels in Canada, England,Denmark, Belgium, Egypt, Israel, Libya the Philippines, Iceland, SouthAfrica and northern Australia immediately following the passage ofseismic waves. An astonishing 7-m fluctuation in a well in South Dakotawas reported (Vohris, 1968). A change about 1 m was reported in a wellin Puerto Rico (Vohris, 1968).

Numerous investigations also show the effect of earthquakes on oilproduction. Steinbrugge and Moram (1954) described variations in oilproduction in Kern county during the Southern California earthquake ofJul. 21, 1952. Several of the wells showed increased casing pressuremany times above normal in the first few days following the earthquake.However, several wells in the same field did not show changes,indicating a complex nature to the effect. One example is cited wheretwo neighbouring wells behaved very differently. One well showed anincreased production from 20 bbl/day to 34 bbl/day immediately after theearthquake, whereas another dropped in production from 54 bbl/day toless than 6 bbl/day.

Simkin and Lopukhv (1989, 14) cite an example from the Starogroznenskoyeoil field in the Northern Caucasus, where production increased by 45%following the earthquake of Jan. 7, 1938.

Summary of Case Studies of Earthquake Influence on Oil Production

Seismic intensity in Epicentral Case Field Earthquake oil field distanceDuration of No Reference location magnitude (12-pt. scale) (km) Observedeffect effect 1 Steinbrugge and Kern 7.6  8-11 80 Mixed effects ofincreased and Moran (1954) Country decreased oil production, Californiaincreased casing pressure 2 Smirnova (1968) Cudermes 3.5 and 4.5 5-710-15 Increased oil Less than a field, 4.5 and 4.2 4-7 10-15 production,largest effect month Caucasus near faults 3 Voytov et al. Different 6.54-7  50-300 Large change in oil several (1972) fields in production,renewed months to Daghestan production in abandoned three years andwells, changes in Caucasus production associated with passive faults 4Osika (1981) Anapa, 4.8 6 30 Increased oil production Northern from somewells, Caucasus pronounced near anticlines, increased oil pressure

A number of publications consider the proposed mechanisms of the effectsof weak elastic waves on saturated media in detail (Bodine, 1954a,1954b, 1955; Duhon, 1964; Surguchev et. al., 1975; Gadiev, 1977;Wallace, 1977; Kuzenetsov and Efimova, 1983; Kissim and Staklianin,1984; Vakhitov and Simkin, 1985; Sadovskiy et. al., 1986; Simkin andLopukhov, 1989; Kuzenetsov and Simkin, 1990; Kissin, 1991; Simkin andSurguchev, 1991).

Fundamentally, gravitational and capillary forces are principallyresponsible for the movement of fluids in a reservoir (Simkin, 1985;Odeh, 1987). Gravitational forces act on the difference in densitybetween the phases saturating the medium, as illustrated in FIG. 4.

The residual oil in a typical depleted reservoir is generally containedin the form of droplets dispersed in water. Density differences inducethe separation of oil from the water, which is a well-known effect ingravitational coalescence. Capillary forces play an important role inliquid percolation through fine pore channels. Liquid films are adsorbedonto pore walls during the percolation process. These films reduce thenormal percolation by reducing the effective diameter of the poretroughs. If the pore is small, the boundary film may block percolationaltogether. Percolation may resume only when some critical pressuregradient is applied. Furthermore, the presence of mineralization in thepercolation fluid changes the thickness of the fluid film. Calculationsshow that the average thickness of the surface film of water in a porouschannel is inversely proportional to the salt concentration, and rangesfrom 5 μm (NaCl solution with a concentration of 100 g/L) to 50 μm(concentration of 1 g/L) (Kuznetsov and Simkin, 1990, p. 123; Fairbanksand Chen, 1971: Dawe et. al., 1987).

In saturated reservoirs, the water and oil phases are intermixed anddispersed within each other. The important attribute of the relativepermeabilities between the phases, which governs the oil yield factor,is the existence of a threshold oil saturation level, So, below whichthe oil is immobile (Odeh, 1987; Nikolaevskiy, 1989). At lower oilsaturation, oil breaks into isolated droplets. As a result, the oilyield of a water-bearing stratum exhibits a physical limit of So=1. Forexample, if So=0.3, then only 70% of the oil can be extracted using itsnatural mobility.

Nikolaevski (1989) speculates that the excitation of elastic waves canchange the phase permeability, thereby increasing the mobility of theoil below So. Elastic wave fields may reduce the influence of capillaryforces on oil percolation considerably, resulting in an increased rateof migration through the porous medium. This appears to explain whyvibration of the surface reduces the adherence of fluid to it.Mechanical vibrations destroy the surface films adsorbed on the poreboundaries, thereby increasing the effective cross-section of the pores.The destruction of films occurs from both weak and intensive wavefields. In the latter case, a number of different non-linear effectsproduced by intense ultrasound such as in-pore turbulence, acousticstreaming and cavitation (Kuznetsov and Simkin, 1990, 126-127) may alsocontribute to this effect. Another effect increasing percolation is thereduction of the surface tension and viscosity of liquids in theultrasonic field, which apparently is caused by heating of the medium asa result of ultrasound absorption (Johnson, 1971).

Low-frequency waves are less likely to produce non-linear elasticeffects because the wave intensity (density of energy flux) isproportional to frequency squared (Nosov, 1965, 5). However, in thepresence of an alternating pressure field whose wavelength exceeds thediameter of oil droplets and gas bubbles in the water, droplets areinduced to move because of their different densities (Kuznetsov et al.,1986; Sadovskiy et al., 1986). A theory describing this effect wasdeveloped by Vakhitov and Simkin (1985, 189-191), and Kuznetsov andSimkin (1990, 220-222). Because gas bubbles usually adhere to thesurface of the oil droplets, they carry oil droplets in response to theoscillatory field (Simkin, 1985).

Bjerknes forces, which are attractive forces acting between theoscillating droplets of one liquid in another, induce the coalescence ofoil droplets (Nosov, 1965, 13; Kuznetsov and Simkin, 1990, 129). Thus,as shown schematically in FIG. 5, continuous streams of oil capable offlow may be formed out of oil droplets dispersed with wave excitation.

Most of the mechanisms involving fluid percolation described above applyto the effects of relatively weak elastic waves. Major mechanismsinvolved in cases of weak and strong excitation seem to be essentiallydifferent. For example, high-density ultrasound is proposed for theprocedures to remove wellbore damage caused by scales and precipitants.The effect produced in this case is purely mechanical destruction oflocal deposits, and has nothing to do with enhanced oil mobility. Whatis missing in the present investigation of the effect of weak elasticwaves on saturated media is a quantitative description of the majormechanisms and the numerical model theory that could predict theresults.

The amount of oil recovered increases with decreasing oil viscosity, andexplains some of the synergy effect with electrical and soundstimulation of the reservoir.

Cherskiy et al. (1997) measured the permeability of core samplessaturated with fresh water in the presence of an acoustic field.According to their description, the permeability of the samplesincreased sharply (by a factor of 82) within a few seconds of thebeginning of the pulse-mode treatment; however, the permeabilitydecreased to the value before the stimulation a few minutes after theacoustic field was turned off.

FIG. 6 shows the results for both pulse- and continuos-wave (cw) modeexcitation as a function of the sound intensity.

The same permeability values were obtained in the pulse mode as in thecontinuous mode, with intensities 10 to 15 times lower. This may beexplained by the continuos mode causing the fluid droplets to oscillate,whereas the pulse mode propagates directed pressure pulses. This effectcan be illustrated by gently knocking on a paper plate with small waterdroplets. The pulses will make the water slide in a direction oppositeto the direction of the pulses.

All mechanical oscillations in a medium will eventually be convertedinto heat by the damping effect. The heat thus released from thevibrations will raise the temperature with a corresponding reduction inthe viscosity and possibly also a partial phase transition (evaporation)of the fluids.

The mechanical force carried by the vibrations may also result in“frictional heat” due to different accelerations of the matrix and thefluids because of their differing densities.

Reduced hydraulic friction near the oil well was reported in workperformed with ultrasonic treatment of an oil well in the Soviet Union.The same effect may also be achieved with low-frequency vibrations bygenerating pink noise where the low-frequency waves are modulatinghigher frequencies oscillations. This results in an absorption of thehigher frequency mode in the well area, giving rise to reduced hydraulicfriction, while the low-frequency mode may continue deeper into theformation and contribute to the effects described above.

C: C. Holloway present the following approach to the effect of sonicstimulation of an oil reservoir:

The minimum pressure gradient required for “snap-off” is calculated asfollows. Darcy's law for the fluid flow rate is:

q/Ar=(k/μ)(dp/dx)

where:

q=flow rate (cm³/sec),

Ar=cross section area (both rock and pores) (cm²),

k=permeability (Darcy),

μ=viscosity (cp), and

dp/dx=pressure gradient (atm/cm).

The flow rate through the cross-sectional area of pores only is:

q/Ap=(k/eμ)(dp/dx)

q=(k/eμ)(dp/dx)*Ap

where:

Ap=cross-sectional area of pores only (cm²) and

e=porosity

The rate of flow through a pore of radius r is:

q=(3.14r ² k/eμ)(dp/dx).

At a frequency of N cycles per second, the time in which the flow canoccur is 1/2N seconds, so the volumetric flow is:

Q=(3.14r ² k/2Neμ)(dp/dx) (cm³).

Imposing the condition for snap-off,

(3.14r ² k/2Neμ)(dp/dx)³ (p/6)(7r)³.

Solving for the required pressure gradient,

dp/dx ³(ReμN/k)[(7)313](atm/cm).

For a grain size of 10 μm, the pressure gradient required for snap-offis

dp/dx=18.9 N (psi/ft).

The minimum pressure gradient were calculated for different frequenciesat 50 m from the stimulated well:

Frequency (Hz) Radius Static 0,0016 0,016 0,16 1,6 16 160 50 m 0,0880,129 0,51 3,1 26 257 2.567

Yenturin A. Sh., Rakhimkulov R. Sh., Kharmanov N. F. (Bash NIPIneft) haspresented the following approach as regard choice of frequencies to workin the formation in the zone adjacent to the well using vibratoryprocesses:

Over the last few years there has been a growing interest for the use ofacoustic fields and wave phenomena to intensify the various processes toextract petroleum and also to increase the extraction index of oil fromthe formations. The reason is the rational use of energy, theconsiderable acceleration and the better performance of sometechnological processes in a wave field. The best prospects are inworking on the formation in the zone adjacent to the well usingvibratory and wave processes, to intensify the oil extraction. In thisway a deeper cleaning of the reservoir rocks and also the most efficientwater injection and other displacing agents of the petroleum areobtained.

The oil extraction index can be increased using a better percolation ofthe water in consequence of the cleaning in the zone adjacent to thewell, with low permeability formations coming into production and with agreater degree of displacement of the petroleum by the water or by otheragents.

One of the fundamental questions for developing techniques that involvewave processes is to determine penetration depths of the acoustic energyin the formation, sufficient to move the fluids in the rock pores. Togenerate wave fields in the zone adjacent to the well hydrodynamicirradiation devices are used that are based on the energy of the flow ofa liquid pumped through them, and also high frequency sonic andultrasonic generators with electrical input (1). Therefore, as theproduction practice shows, the hydrodynamic devices and sonic generatorsdo not always obtain a positive effect, specially in injection wells.This is explained, firstly, by the fact that when establishing the basicparameters of the generators, the frequency and intensity of theacoustic field that must be determined in the concrete conditions of thedeposit are not always taken into consideration. For this reason it isof practical interest to study the effective penetration depth of theacoustic waves in the formation.

There, basically, two methods to increase the oil extraction index usingacoustic fields. The first is summarized in provoking vibrations in theformation itself, for example using seismic acoustic waves. In this casethe oscillation energy in the elementary mass dM is determined by theequation:

dE=0,5*{overscore (ω)}² A ² ΔM

where {overscore (ω)}—frequency of the vibrations; A—range of thedisplacements.

Consequently to generate vibrations in the rock, a very strong energy isneeded which makes this method difficult to do.

It is the second method that has better prospects, which is based on thegeneration of hydrodynamic pressure waves in a fluid and their spreadthrough the formation pores. We shall examine this method in moredetail. The most common productive formations have pores with diameter rthat varies, predominantly, between 1 and 10 micra ({fraction (1/1000)}mm). Due to the existence of friction forces between the liquid and thewalls of the pores, the formation presents attenuating properties inrelation to the hydrodynamic waves, and when choosing the acousticfield, one of the determining factors can be the effective penetrationdepth of these waves in the rock.

The spread of the energy from the vibrations through the internalfriction of the liquid and its thermal conductivity is relatively smallif compared to the dispersjon caused by the friction next to the wall ofthe pore channels (2). For example, the range of the plane wave in waterat frequency 3 MHz becomes only 10 times less at a distance of 10meters. For this reason, the known equations of the acoustic (movement,continuity and state) can be presented in the following manner (2):

−δp/δx=ρδu/δt+ραu ² ,−ρp/δt=pcδu/δx  1

where p, u—hydrostatic pressure and displacement speed; x—distance;t—time; ρ-density of the liquid; α=λ/8δ; c—speed of the sound in theliquid; λ—coefficient of hydraulic resistance; δ=F/κ—hydraulic radius ofthe flow section (for round channels 5=0.5 r); F—flow area; κ—soakableperimeter.

For a porous medium

λ=2ν/v _(φ)(m/k)^(0,5)  2

where ν—kinematic viscosity of the liquid; m—coefficient of the rockporosity; v_(φ)—filtration speed; k-roch permeability.

The filtration speed has the components static (in the calculations, tomake it simpler, we assume that it is constant in the x length) and μvariable. If there is a need to take into consideration the internallosses in the liquid in the system 1 in a linear form c is substitutedby the complex speed of the sound.

For the harmonic zones the Fourier transformation can be used in theform u=Ue^(j){overscore (ω)}^(t). Then, reducing system 1 and a waveequation, we will have, after the elementary transformations:

 −{overscore (ω)}²U+jn{overscore (ω)}U=c ² d ²U/dx ² ,n=νm/2r(m/k)^(0,5)  3

The limitrophe equations for the equation 3 has the form u=0 beingU=U_(o)(U_(o)—range of the alternate displacement at the opening of thewell, caused by the hydrodynamic generator) dU/dx=0.

Equation 3 is a linear differential equation of a well-known kind. Usingthe Laplace transformations in sequence and the algebraictransformations, we shall have the final result of the equation 3 in theform:

U(x)=U_(o)(sh ² αβx+cos 2βx)^(0,5) exp(−tgαxtgβx)  5

Where:

α=α₁; β=α₂; α₁ =ω/c2^(0,5)[(1+n2/ω2)^(0,5)+(−1)^(i))]^(0,5)

(i=1, 2)

Passing from U to the hydrodynamic pressure p, U=p/ρc substitution isused.

In FIG. 7, the results of the calculations appear (using the equation 5)for the hydrodynamic pressure losses, . . . Δp_((L))/Δp_(o)≅U_((L))U_(o)in relation to the length of the pore channel L, as well as to losesrelating to energy ∈_((L))/∈_(o)≅U² _((L))/U_(o) ².

As shown in the figure, the effective penetration depth of theultrasonic waves with a frequency of 2.10⁴-10¹⁰ Hz is no greater than1-2 cm. Consequently, the ultrasonic waves are only usable for a not sodeep acoustic treatment in the formation in the zone adjacent to thewell.

The low frequency waves (20-40 Hz) can be used for the treatment down toa 1-2.5 m penetration depth. For a deeper hydrodynamic treatment it isrecommend to use a generator with infrasonic frequencies (0.5-5 Hz). Sotests carried out at the UNI on sandstone samples with permeabilities of0.115-0.16 μ² made it possible to obtain a reduction in the residualpetroleum of 11.6-32.3% with vibrations at the frequency of 2-4 Hz andpressure range of 2-20 MPa (smaller residual petroleum indices were seenin rocks with less permeability).

For a greater increase in the petroleum extraction index we can considerthat the most efficient waves are the sub-infrasonic hydrodynamic ones(frequency less than 0.5 Hz). Among the latter the cyclicle pumpings canbe considered, that produce an increase in the petroleum extractionindex (the frequency of the cycles is less than 2.10⁻⁶ Hz).

When the wave processes are applied to heterogenous concrete layers, thehydrodynamic effect can be intensified diverting the waves to the sideof the low permeability layers, which is managed through the priorplugging of the more permeable rocks. It is expected that this combinedeffect must be more effective with lower frequency waves.

When choosing the acoustic fields, we must take into consideration thatthe subinfrasonic waves differ to only a slight degree of attenuationand dispersjon when passing through the pipe. Thus for their generation,automatic hydrodynamic generators can be used on the surface, whichtogether with the rational control of the pumping system in groups ofwells and by using computers, may increase the petroleum extractionindex of the fields.

Based on what we have already presented about electrical and sonicstimulation of an oil reservoir, we have found that the EEOR electricand sonic methods give a positive synergy effect when applied together.The main reason is believed to be that as the electrical stimulationbreaks the surface tension and reduces the thermal viscosity, it favoursthe effect of the acoustic stimulation to a much larger extent than whenthe vibrations are applied alone.

This synergy effect not only increases the yield of the oil but alsogives a greater production flow, and thus reduces the energy costs perunit of oil produced.

To verify this idea, two identical artificial oil reservoirs wereconstructed of a sandstone from an outcrop in Bahia, Brazil, with apermeability of 500 mD. The sandstone was coated in reinforced epoxy andwas equipped with three production wells and injection wells for thewater drive. The reservoirs were filled with water and crude oil. Thesewere the ninth and tenth tests performed in Brazil.

In the first test, the reservoir was depleted using the water drivewithout any stimulation until we reached a water breakthrough in theproducing wells. The reservoir was then stimulated with electricity andvibration simultaneously.

In the second test, the reservoir was stimulated from the start usingthe complete EEOR process. The results are presented in the graph ofFIG. 8, and show clearly the increased production flow in the secondtest, which clearly shows the synergy effect of the process.

As have now been explained above regarding electric and acousticstimulation by known methods, one can observe that the energy from thedifferent stimuli is dissipated from the well in accordance to alogarithmic scale.

To improved the oil recovery in addition to the above mentioned methods,it would be advantageous to make it possible to have the energydissipated at a wider area from the well, but also the gain an in-situvibration effect different from the one described above.

One way to obtain this would be to have several vibrators extendingradically out from the well bore. But, this is an impossible task.

We have thus looked at other possibility to create a vibration mediumfrom the well bore and which will be described as follows:

One standard operation in well completion and after the well has beencompleted, is to perform so-called fracturing of the reservoir by sandmixed in water and certain chemicals to aid the penetration of the sandinto the reservoir.

The fracturing job is done by that the mixture of sand, water andchemicals are injected into the well and by a certain pressure, themixture is pressed into the formation. This can be observed as a suddendrop in the pumping pressure at the surface. Normal facturing jobs canfracture up to 400 feet into the formation.

Now, as we know that an alternating electric field is to be passed fromthe wells and into the formation, it is thus possible to have theelectrical current affecting a substance which will respond mechanicallyto the alternating current. Such a substance would be any magneticmaterial such as magnetite, small ceramic and metallic magnets etc. Butin addition to such materials, other electrostrictive materials can beapplied such as “Terfenol” which is an alloy of Ferrum, Terbium andDysprosium. Other such materials can be piezoelectric minerals, alloysof rare earth metals or other similar organic or inorganic materials.Further more, the textbook <<The Application of Ferroelectric Polymers>>by T. T. Wang, J. M. Herbert and A. M. Glass describes a number of suchmaterials which can be used in combination with a fracturing operation.

When applied to an alternating electric field, these substances willvibrate in-situ at the same frequency as the applied alternatingcurrent. Because of the small unit mass of the single particle, it ispossible to change the frequency of the alternating current to match thebest response of the vibration to the production.

Accordingly, a method of performing the invention would includeperforming a “fracturing” of the reservoir by injecting a substanceincluding any magnetic or electrostrictive material into the well andinto its adjacent formation, applying an alternating electric field fromthe well that has been used for injecting the substance into theformation, and changing the frequency of the alternating current tomatch the best response of the vibration to substantially reduce surfacetension and assist in keeping formation pores open for fluid flow.

The invention described above is provided by way of illustration andshould not be construed to limit the invention Those skilled in the artwill readily recognize various modifications and changes which may bemade to the present invention without strictly following theapplications and methods illustrated and described herein, and withoutdeparting from the true spirit and scope of the present invention whichis set forth in the following claims.

These in-situ micro-vibrations will contribute to a substantialreduction of the surface tension, but will also aid in keeping the poresopen for the fluid flow.

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What is claimed is:
 1. A method to increase the oil production from anoil reservoir, wherein a magnetic and/or magnetostrictive material isinjected into the oil reservoir through an oil well and is put intovibration by the aid of an alternating electric field provided throughthe oil well, and the vibrations of the material are changeable bychanging a frequency of the alternating electric field, and thevibrations disrupt the surface tension of oil and water to improve theflow of oil towards the oil well.
 2. A method in accordance with claim1, wherein the magnetic or magnetostrictive material is magnetite,hematite, steel sand or alloys of me earth metals.
 3. A method forincreasing oil production from an oil reservoir, the method comprising:injecting a magnetic or magnetostrictive material into an oil reservoirthrough an oil well; applying an alternating electric field to theinjected material through the oil well; vibrating the injected materialwith the electric field to disrupt the surface tension of oil and waterto improve the flow of oil towards the oil well, the vibrations beingchangeable by changing a frequency of the electric field; removing oilfrom the oil reservoir through the same oil well.